Zoom lens system

ABSTRACT

A zoom lens system comprising a front lens unit which is composed of a positive meniscus lens component having a convex surface on the object side, a negative lens component and at least one positive lens component, and has a positive refractive power as a whole, and a rear lens unit which is composed of a positive lens component, a biconcave lens component and a negative meniscus lens component having a convex surface on the image side, and has a negative refractive power as a whole; and adapted so as to perform variation of focal length by varying the airspace reserved between said front lens unit and said rear lens unit.

BACKGROUND OF THE INVENTION

a) Field of the invention

The present invention relates to a zoom lens system which has a shorttotal length and is suited for use especially with lens shutter cameras,etc.

b) Description of the prior art

As the conventional zoom lens system which has a range of field anglescovering the standard field angle (on the order of 2ω=47°) and avari-focal ratio on the order of 2, and is designed so compact as to bebuilt in the lens shutter cameras, there is known the zoom lens systemcomposed of a front lens unit having a positive refractive power andanother lens unit having a negative refractive power.

This zoom lens system is characterized in that it has a compositionsimilar to that of the telephoto type lens system, thereby permittingshortening the total length thereof. As a conventional example of thezoom lens system having the composition described above, there is knownthe lens system disclosed by Japanese Patent Kokai Publication No. Sho64-52111 (U.S. Pat. No. 4,838,669) which has aberrations corrected withgood balance and a shortened total length.

This type of zoom lens system has a short back focal length andcomprises a rear lens unit located close to the image surface, therebyhaving a defect that the lens components arranged in the rear lens unithave large diameters. Speaking concretely, the lens components arrangedin the rear lens unit have diameters which are approximately twice aslarge as those of the lens components arranged in the front lens unit,thereby enhancing manufacturing cost of the lens components arranged inthe rear lens unit.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a compact zoomlens system which covers the standard field angle, has a vari-focalratio on the order of 2 and favorably corrected aberrations, and can bemanufactured at a low cost.

The zoom lens system according to the present invention comprises afront lens unit having a positive refractive power and a rear lens unithaving a negative refractive power, and adapted so as to performvariation of focal length by varying the airspace reserved between theselens units. In the zoom lens system according to the present invention,the front lens unit comprises, in the order from the object side, apositive meniscus lens component having a convex surface on the objectside, a negative lens component and at least one positive lenscomponent, and the rear lens unit comprises, in the order from theobject side, a positive lens component, a biconcave lens component and anegative meniscus lens component having a convex surface on the imageside. The zoom lens system according to the present invention ischaracterized in that the positive lens component and the biconcave lenscomponent arranged in said rear lens unit are made of plastic materials,said plastic lens components comprise at least one aspherical surfacewhich has positive refractive power strengthened as the surface portionsare farther from the optical axis, and that the rear lens unit isdesigned so as to satisfy the following conditions (1) and (2):

    -1 <(r.sub.II2a +r.sub.II2b)/(r.sub.II2a -r.sub.2b) <0.2   (1)

    -0.15 <f.sub.II /f.sub.II12 <0.15                          (2)

wherein the reference symbols r_(II2a) and r_(II2b) represent the radiiof curvature on the object side surface and the image side surfacerespectively of the biconcave lens unit, the reference symbol f_(II)designates the focal length of said rear lens unit, and the referencesymbol f_(II12) denotes the total focal length of the two plastic lenscomponents.

In the zoom lens system according to the present invention, curvature offield is corrected by arranging at least one aspherical surface in therear lens unit wherein the axial ray and the offaxial ray are separatedfrom each other.

The condition (1) defines the shape of the biconcave lens componentarranged in the rear lens unit. If the upper limit of the condition (1)is exceeded, said biconcave lens component will have too weak arefractive power and cannot correct the curvature of field sufficiently.If the lower limit of the condition (1) is exceeded, said lens componentwill not be the biconcave lens component but will have a meniscus shape,thereby shortening the distance as measured from the final surface ofthe zoom lens system to the image surface. That is to say, when saidlens component has the meniscus shape, the rear principal point of thezoom lens system is shifted toward the object side, thereby bringing thefinal surface of the zoom lens system closer to the image surface andenlarging the diameter of the lens component arranged on the most imageside.

The condition (2) is adopted for shortening the distance of the shift ofthe focal point of the zoom lens system which is caused due tovariations of temperature and humidity especially of the plastic lenscomponents. Speaking concretely, when the total refractive power of thepositive plastic lens component and the negative plastic lens componentis weakened, these lens components will have refractive powers which arenearly equal to each other in the absolute values thereof and, since thevariations of temperature and humidity cause variations of therefractive powers of the positive lens component and the negative lenscomponent which are in the directions reverse to each other and nearlyequal to each other in the absolute values thereof, the total refractivepower will be scarcely varied and the focal point of the zoom lenssystem is shifted only for a very short distance due to the variationsof temperature and humidity.

If the upper limit or the lower limit of the condition (2) is exceeded,variations of temperature and humidity will remarkably vary the totalrefractive power of the two plastic lens components and shift the focalpoint of the zoom lens system for a long distance, thereby bringing thezoom lens system out of focus.

In order to correct aberrations more favorably in the zoom lens systemaccording to the present invention, it is desirable to specify the upperlimit of the above-mentioned condition (2) at 0.03. That is to say, itis more desirable to design the rear lens unit so as to satisfy thefollowing condition (2'):

    -0.15 <f.sub.II /f.sub.II12 <0.13                          (2)

If the upper limit of the condition (2') is exceeded, the focal point ofthe zoom lens system will be shifted for a shorter distance by thevariations of temperature and humidity, but it will be difficult toproperly balance aberrations with one another in the zoom lens system orobtain good image quality.

For the zoom lens system according to the present invention, it isdesirable to fabricate the two plastic lens component from one and thesame material, and design the rear lens unit so as to satisfy thefollowing condiitons (3) through (5):

    1 ≦|f.sub.II1 /f.sub.II |≦4 (3)

    1 ≦|f.sub.II2 /f.sub.II |≦4 (4)

    -0.8 <(r.sub.II1b -r.sub.II2a)/(r.sub.II1b +r.sub.II2a) <0 (5)

wherein the reference symbol f_(II) represents the focal length of therear lens unit, the reference symbol f_(II1) designates the focal lengthof the positive plastic lens component arranged in the rear lens unit,the reference symbol f_(II2) denotes the focal length of the biconcaveplastic lens component arranged in the rear lens unit, the referencesymbol r_(II1b) represents the radius of curvature on the image sidesurface of the positive lens component arranged in the rear lens unitand the reference symbol r_(II2a) designates the radius of curvature onthe object side surface of the biconcave plastic lens component arrangedin the rear lens unit.

These two plastic lens components should desirably be fabricated fromone and the same material so that the variations of the characteristicsof these lens components due to variations of temperature will becancelled with each other. When these lens components are fabricatedfrom one and the same material, the condition (2) exhibits a highereffect to shorten the distance of the shift of the focal point of thezoom lens system to be caused by variations of temperature and humidity.

The conditions (3) and (4) define the focal lengths of the plastic lenscomponents arranged in the rear lens unit. If the lower limit of thecondition (3) or (4) is exceeded, the refractive powers of the positiveplastic lens component and the negative plastic lens component will bevaried remarkably due to variation of temperature, thereby allowing thetotal focal length of these positive and negative plastic lenscomponents to be varied due to temperature variation. If the upper limitof the condition (3) or (4) is exceeded, these plastic lens componentswill produce aberrations in smaller mounts or contribute less tocorrection of aberrations with good balance in the zoom lens system as awhole, thereby making it impossible to correct aberrations with goodbalance in the zoom lens system as a whole.

The condition (5) defines the shape of the air lens formed between thetwo plastic lens components. If the lower limit of the condition (5) isexceeded, spherical aberration of high orders will be producedremarkably on the negative side. If the upper limit of the condition (5)is exceeded, the positive plastic lens component will produceaberrations in too small amounts relative to those of the aberrationsproduced in the rear lens unit, thereby making it impossible to correctaberrations favorably in the zoom lens system as a whole.

For the zoom lens system according to the present invention, it isdesirable to compose the front lens unit, in the order from the objectside, of a positive meniscus lens component having a convex surface onthe object side, a negative lens component and two positive lenscomponents having convex surfaces on the image side, and further designthe front lens unit so as to satisfy the following condition (6):

    -2.4 <(r.sub.I2b -r.sub.I3a)/(r.sub.I2b +r.sub.I3a)        (6)

wherein the reference symbol r_(I2b) represents the radius of curvatureon the image side surface of the negative lens component arranged in thefront lens unit and the reference symbol r_(I3a) designates the radiusof curvature on the object side surface of the positive lens componentwhich is arranged on the image side of said negative lens component andadjacent thereto.

The condition (6) defines the shape of the air lens formed between thenegative lens component and the positive lens component which isarranged on the negative lens component and adjacent thereto in thefront lens unit. If the lower limit of the condition (6) is exceeded,spherical aberration of high orders will be produced remarkably on thepositive side. This spherical aberration of high orders will be producedmore remarkably at the tele position.

Further, it is more desirable, from the viewpoint of manufacturing ofthe two lens components forming the air lens, to add an upper limit tothe condition (6) so as to define the following condition (6') anddesign the front lens unit so as to satisfy the condition (6'):

    -2.4 <(r.sub.I2b -r.sub.I3a)/(r.sub.i2b +r.sub.i3a) <-0.5  (6)

If the upper limit of the condition (6') is exceeded, the opticalperformance of the zoom lens system will be largely influenced bymanufacturing errors of the negative lens component and the positivelens component which form said air lens, thereby making it necessary tospecify stricter precision for molding and assembly of the parts of thezoom lens system or enhancing manufacturing cost of the zoom lenssystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 through FIG. 5 show sectional views illustrating compositions ofEmbodiments 1 through 5 of the zoom lens system according to the presentinvention;

FIG. 6, FIG. 7 and FIG. 8 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition of the Embodiment 1 of the present invention;

FIG. 9, FIG. 10 and FIG. 11 show graphs illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition of the Embodiment 2 of the present invention;

FIG. 12, FIG. 13 and FIG. 14 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition of the Embodiment 3 of the present invention;

FIG. 15, FIG. 16 and FIG. 17 show curves illustrating aberrationcharacteristics at the wide position, intermediate focal length and teleposition of the Embodiment 4 of the present invention; and

FIG. 18, FIG. 19 and FIG. 20 show graphs visualizing aberrationcharacteristics at the wide position, intermediate focal length and teleposition of the Embodiment 5 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described more detailedly below withreference to the preferred embodiments illustrated in the accompanyingdrawings and given in the form of the following numerical data:

    ______________________________________                                        Embodiment 1                                                                  ______________________________________                                        f = 39.348˜77.196 mm, F/4.65˜F/6.62                               2ω = 148.690°˜122.398°                              ______________________________________                                        r.sub.1 = 20.7232                                                                       d.sub.1 = 2.3325                                                                            n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.84                         r.sub.2 = 86.5597                                                                       d.sub.2 = 1.5167                                                    r.sub.3 = -22.5439                                                                      d.sub.3 = 3.1266                                                                            n.sub.2 = 1.83400                                                                        ν.sub.2 = 37.16                         r.sub.4 = 40.6080                                                                       d.sub.4 = 2.9170                                                    r.sub.5 = -603.1115                                                                     d.sub.5 = 2.5863                                                                            n.sub.3 = 1.58313                                                                        ν.sub.3 = 59.36                         r.sub.6 = -18.4149                                                                      d.sub.6 = 0.1500                                                    r.sub.7 = 33.0165                                                                       d.sub.7 = 2.5265                                                                            n.sub.4 = 1.50137                                                                        ν.sub.4 = 56.40                         r.sub.8 = -44.5971                                                                      d.sub.8 = 0.8000                                                    r.sub.9 = ∞ (stop)                                                                d.sub.9 = D (variable)                                              r.sub.10 = -65.8544 (aspherical surface)                                                d.sub.10 = 2.5259                                                                           n.sub.5 = 1.50050                                                                        ν.sub.5 = 56.68                         r.sub.11 = -21.6372                                                                     d.sub.11 = 0.3575                                                   r.sub.12 = -40.4041                                                                     d.sub.12 = 1.3127                                                                           n.sub.6 = 1.50050                                                                        ν.sub.6 = 56.68                         r.sub.13 = 176.9470                                                                     d.sub.13 = 6.0356                                                   r.sub.14 = -12.4911                                                                     d.sub.14 = 1.6000                                                                           n.sub.7 = 1.74100                                                                        ν.sub.7 = 52.68                         r.sub.15 = -30.2214                                                           ______________________________________                                        aspherical surface coefficients                                               K = 1.0000, A = 0, B = 0.13944 × 10.sup.-4                              C = 0.69628 × 10.sup.-7, D = -0.46929 × 10.sup.-12                f      39.348         55.104  77.196                                          D      14.390          7.747   3.000                                          f.sub.B                                                                               7.546         23.372  45.559                                          (r.sub.II.spsb.2.sub.a +  r.sub.II.spsb.2.sub.b)/(r.sub.II.spsb.2.sub.a -     r.sub.II.spsb.2.sub.b) = -0.628                                               f.sub.II /f.sub.II.spsb.12 = -0.015, |f.sub.II.spsb.1 /f.sub.II      |  = 2.085                                                           |f.sub.II.spsb.2 /f.sub.II |  = 2.165,                      (r.sub.II.spsb.1.sub.b - r.sub.II.spsb.2.sub.a)/(r.sub.II.spsb.1.sub.b +      r.sub.II.spsb.2.sub.a) = -0.302                                               (r.sub.I.spsb.2.sub.b - r.sub.I.spsb.3.sub.a)/(r.sub.I.spsb.2.sub.b +         r.sub.I.spsb.3.sub.a) = -1.144                                                ______________________________________                                    

    ______________________________________                                        Embodiment 2                                                                  ______________________________________                                        f = 39.332˜73.335 mm, F/3.997˜F/7.452                             2ω = 147.130˜122.378                                              ______________________________________                                        r.sub.1 = 17.9351                                                                       d.sub.1 = 2.9960                                                                            n.sub.1 = 1.56384                                                                        ν.sub.1 = 60.69                         r.sub.2 = 102.4003                                                                      d.sub.2 = 1.4236                                                    r.sub.3 = -21.1329                                                                      d.sub.3 = 3.0186                                                                            n.sub.2 = 1.83400                                                                        ν.sub.2 = 37.16                         r.sub.4 = 38.1165                                                                       d.sub.4 = 1.1754                                                    r.sub.5 = -496.2130                                                                     d.sub.5 = 4.0961                                                                            n.sub.3 = 1.48749                                                                        ν.sub.3 = 70.20                         r.sub.6 = -16.1071                                                                      d.sub.6 = 0.7166                                                    r.sub.7 = 35.9373                                                                       d.sub.7 = 2.6990                                                                            n.sub.4 = 1.58267                                                                        ν.sub.4 = 46.33                         r.sub.8 = -37.5401                                                                      d.sub.8 = 1.8706                                                    r.sub.9 = ∞ (stop)                                                                d.sub.9 = D (variable)                                              r.sub.10 = -72.2003                                                                     d.sub.10 = 2.9984                                                                           n.sub.5 = 1.50050                                                                        ν.sub.5 = 56.68                         r.sub.11 = -24.4040 (aspherical surface)                                                d.sub.11 = 0.1945                                                   r.sub.12 = -64.1976                                                                     d.sub.12 = 1.2972                                                                           n.sub.6 = 1.50050                                                                        ν.sub.6 = 56.68                         r.sub.13 = 109.3869                                                                     d.sub.13 = 6.0675                                                   r.sub.14 = -12.8938                                                                     d.sub.14 = 1.5986                                                                           n.sub.7 = 1.77250                                                                        ν.sub.7 = 49.66                         r.sub.15 = -35.0060                                                           ______________________________________                                        aspherical surface coefficients                                               K  - 1.0000, A = 0, B = 0.18529 × 10.sup.-4                             C = 0.72277 × 10.sup.-8, D = -0.81442 × 10.sup.-9                 f      39.332         55.160  73.335                                          D      12.924          6.717   2.894                                          f.sub.B                                                                               7.447         22.446  39.668                                          (r.sub.II.spsb.2.sub.a + r.sub.II.spsb.2.sub.b)/(r.sub.II.spsb.2.sub.a -      r.sub.II.spsb.2.sub.b) = -0.260                                               f.sub.II /f.sub.II.spsb.12 = -0.039, |f.sub.II.spsb.1 /f.sub.II      |  = 2.541                                                           |f.sub.II.spsb.2 /f.sub.II |  = 2.840,                      (r.sub.II.spsb.1.sub.b - r.sub.II.spsb.2.sub.a)/(r.sub.II.spsb.1.sub.b +      r.sub.II.spsb.2.sub.a) = -0.449                                               (r.sub.I.spsb.2.sub.b - r.sub.I.spsb.3.sub.a)/(r.sub.I.spsb.2.sub.b +         r.sub.I.spsb.3.sub.a) = -1.166                                                ______________________________________                                    

    ______________________________________                                        Embodiment 3                                                                  ______________________________________                                        f = 41.425˜77.191 mm, F/3.552˜F/6.619                             2ω = 148.688°˜124.852°                              ______________________________________                                        r.sub.1 = 20.6545                                                                       d.sub.1 = 3.0988                                                                            n.sub.1 = 1.69680                                                                        ν.sub.1 = 56.49                         r.sub.2 = 477.6797                                                                      d.sub.2 = 1.4515                                                    r.sub.3 = -23.4282                                                                      d.sub.3 = 1.0613                                                                            n.sub.2 = 1.83400                                                                        ν.sub.2 = 37.16                         r.sub.4 = 38.3809                                                                       d.sub.4 = 3.8404                                                    r.sub.5 = -150.0055                                                                     d.sub.5 = 2.4903                                                                            n.sub.3 = 1.50137                                                                        ν.sub.3 = 56.40                         r.sub.6 = -16.3299                                                                      d.sub.6 = 0.1500                                                    r.sub.7 = 56.2737                                                                       d.sub.7 = 3.0000                                                                            n.sub.4 = 1.65830                                                                        ν.sub.4 = 53.44                         r.sub.8 = -59.1896                                                                      d.sub.8 = 0.8000                                                    r.sub.9 = ∞ (stop)                                                                d.sub.9 = D (variable)                                              r.sub.10 = -67.9217                                                                     d.sub.10 = 2.8000                                                                           n.sub.5 = 1.50050                                                                        ν.sub.5 = 56.68                         r.sub.11 = -21.1147 (aspherical surface)                                                d.sub.11 = 0.2200                                                   r.sub.12 = -36.7614                                                                     d.sub.12 = 1.2000                                                                           n.sub.6 = 1.50050                                                                        ν.sub.6 = 56.68                         r.sub.13 = 161.1736                                                                     d.sub.13 = 5.9516                                                   r.sub.14 = -12.8637                                                                     d.sub.14 = 1.6000                                                                           n.sub.7 = 1.77250                                                                        ν.sub.7 = 49.66                         r.sub.15 = -26.4900                                                           ______________________________________                                        aspherical surface coefficients                                               K = 1.0000, A = 0, B = -0.17451 × 10.sup.-4                             C = 0.44891 × 10.sup.-7, D = -0.12213 × 10.sup.-8                 f      41.425         55.107  77.191                                          D      15.072          8.603   3.000                                          f.sub.B                                                                               7.533         21.818  44.874                                          (r.sub.II.spsb.2.sub.a + r.sub.II.spsb.2.sub.b)/(r.sub.II.spsb.2.sub.a -      r.sub.II.spsb.2.sub.b) = -0.629                                               f.sub.II /f.sub.II.spsb.12 = 0.007, |f.sub.II.spsb.1 /f.sub.II       |  = 1.788                                                           |f.sub.II.spsb.2 /f.sub.II |  = 1.778,                      (r.sub.II.spsb.1.sub.b - r.sub.II.spsb.2.sub.a)/(r.sub.II.spsb.1.sub.b +      r.sub.II.spsb.2.sub.a) = -0.270                                               (r.sub.I.spsb.2.sub.b - r.sub.I.spsb.3.sub.a)/(r.sub.I.spsb.2.sub.b +         r.sub.I.spsb.3.sub.a) = -1.688                                                ______________________________________                                    

    ______________________________________                                        Embodiment 4                                                                  ______________________________________                                        f = 39.370˜73.352 mm, F/4.000˜F/7.453                             2ω = 147.136˜122.424                                              ______________________________________                                        r.sub.1 = 18.8903                                                                       d.sub.1 = 2.9999                                                                            n.sub.1 = 1.61272                                                                        ν.sub.1 = 58.75                         r.sub.2 = 137.7096                                                                      d.sub.2 = 1.3977                                                    r.sub.3 = -19.7917                                                                      d.sub.3 = 2.1268                                                                            n.sub.2 = 1.83400                                                                        ν.sub.2 = 37.16                         r.sub.4 = 42.6413                                                                       d.sub.4 = 2.3807                                                    r.sub.5 = -398.6859                                                                     d.sub.5 = 2.9998                                                                            n.sub.3 = 1.49831                                                                        ν.sub.3 = 65.03                         r.sub.6 = -14.8689                                                                      d.sub.6 = 0.1498                                                    r.sub.7 = 35.7759                                                                       d.sub.7 = 2.6999                                                                            n.sub.4 = 1.54072                                                                        ν.sub.4 = 47.20                         r.sub.8 = -47.1805                                                                      d.sub.8 = 1.6400                                                    r.sub.9 = ∞ (stop)                                                                d.sub.9 = D (variable)                                              r.sub.10 = -112.5633                                                                    d.sub.10 = 2.9997                                                                           n.sub.5 = 1.50050                                                                        ν.sub.5 = 56.68                         r.sub.11 = -21.7957 (aspherical surface)                                                d.sub.11 = 0.1995                                                   r.sub.12 = -40.7167                                                                     d.sub.12 = 1.3497                                                                           n.sub.6 = 1.50050                                                                        ν.sub.6 = 56.68                         r.sub.13 = 108.9122                                                                     d.sub.13 = 5.3868                                                   r.sub.14 = -13.8486                                                                     d.sub.14 = 1.5999                                                                           n.sub.7 = 1.69100                                                                        ν.sub.7 = 54.84                         r.sub.15 = -48.4057                                                           ______________________________________                                        aspherical surface coefficients                                               K = 1.0000, A = 0, B = -0.76641 × 10.sup.-5                             C = 0.18777 × 10.sup.-6, D = -0.18097 × 10.sup.-8                 f      39.370         54.015  73.352                                          D      11.149          4.791   0.284                                          f.sub.B                                                                               8.015         22.716  42.130                                          (r.sub.II.spsb.2.sub.a +  r.sub.II.spsb.2.sub.b)/(r.sub.II.spsb.2.sub.a -     r.sub.II.spsb.2.sub.b) = -0.456                                               f.sub.II /f.sub.II.spsb.12 = -0.054, |f.sub.II.spsb.1 /f.sub.II      |  = 1.754                                                           |f.sub.II.spsb.2 /f.sub.II |  = 1.939,                      (r.sub.II.spsb.1.sub.b - r.sub.II.spsb.2.sub.a)/(r.sub.II.spsb.1.sub.b +      r.sub.II.spsb.2.sub.a) = -0.303                                               (r.sub.I.spsb.2.sub.b - r.sub.I.spsb.3.sub.a)/(r.sub.I.spsb.2.sub.b +         r.sub.I.spsb.3.sub.a) = -1.240                                                ______________________________________                                    

    ______________________________________                                        Embodiment 5                                                                  ______________________________________                                        f = 39.343˜73.324 mm, F/3.997˜F/7.450                             2ω = 147.124°˜122.392°                              ______________________________________                                        r.sub.1 = 19.4615                                                                       d.sub.1 = 3.0000                                                                            n.sub.1 = 1.61272                                                                        ν.sub.1 = 58.75                         r.sub.2 = 135.6113                                                                      d.sub.2 = 1.4303                                                    r.sub.3 = -19.1830                                                                      d.sub.3 = 2.2000                                                                            n.sub.2 = 1.83400                                                                        ν.sub.2 = 37.16                         r.sub.4 = 46.1096                                                                       d.sub.4 = 1.0000                                                    r.sub.5 = -230.8738                                                                     d.sub.5 = 4.5068                                                                            n.sub.3 = 1.49831                                                                        ν.sub.3 = 65.03                         r.sub.6 = -14.9166                                                                      d.sub.6 = 0.1500                                                    r.sub.7 = 35.9315                                                                       d.sub.7 = 2.7000                                                                            n.sub.4 = 1.54072                                                                        ν.sub.4 = 47.20                         r.sub.8 = -39.9827                                                                      d.sub.8 = 1.5999                                                    r.sub.9 = ∞ (stop)                                                                d.sub.9 = D (variable)                                              r.sub.10 = -60.7536                                                                     d.sub.10 = 3.0000                                                                           n.sub.5 = 1.50050                                                                        ν.sub.5 = 56.68                         r.sub.11 = -21.3976                                                                     d.sub.11 = 0.2000                                                   r.sub.12 = -46.4027 (aspherical surface)                                                d.sub.12 = 1.3500                                                                           n.sub.6 = 1.50050                                                                        ν.sub.6 = 56.68                         r.sub.13 = 188.2538                                                                     d.sub.13 = 5.6966                                                   r.sub.14 = -12.8386                                                                     d.sub.14 = 1.6000                                                                           n.sub.7 = 1.69100                                                                        ν.sub.7 = 54.84                         r.sub.15 = -38.7289                                                           ______________________________________                                        aspherical surface coefficients                                               K = 1.0000, A = 0, B = 0.13412 × 10.sup.-4                              C = -0.20830 × 10.sup.-7, D =  0.95091 × 10.sup.-9                f      39.343         53.771  73.324                                          D      13.289          7.086   2.575                                          f.sub.B                                                                               7.814         22.275  41.875                                          (r.sub.II.spsb.2.sub.a +  r.sub.II.spsb.2.sub.b)/(r.sub.II.spsb.2.sub.a -     r.sub.II.spsb.2.sub.b) = -0.605                                               f.sub.II /f.sub.II.spsb.12 = -0.058, |f.sub.II.spsb.1 /f.sub.II      |  = 2.132                                                           |f.sub.I.spsb.I2 /f.sub.II |  = 2.459,                      (r.sub.II.spsb.1.sub.b - r.sub.II.spsb.2.sub.a)/(r.sub.II.spsb.1.sub.b +      r.sub.II.spsb.2.sub.a) = -0.369                                               (r.sub.I.spsb.2.sub.b - r.sub.I.spsb.3.sub.a)/(r.sub.I.spsb.2.sub.b +         r.sub.I.spsb.3.sub.a) = -1.499                                                ______________________________________                                    

wherein the reference symbols r₁, r₂,... represent the radii ofcurvature on the surfaces of the respective lens components, thereference symbols d₁, d₂, ... designate the thicknesses of therespective lens components and the airspaces reserved therebetween, thereference symbols n₁, n₂,... denote the refractive indices of therespective lens components, and the reference symbols ν₁, ν₂,...represent the Abbe's numbers of the respective lens components.

In the embodiments described above, the lens components which have n₅,ν₅ and n₆, ν₆ are fabricated from a plastic material.

When the direction of the optical axis is taken as the z axis, thedirection perpendicular to the optical axis is taken as the y axis andthe radius of curvature on an aspherical surface of interest as measuredin the vicinity of the optical axis is represented by R, the shapes ofthe aspherical surfaces used in the embodiments described above areexpressed by the following formula: ##EQU1## wherein the referencesymbol k represents the conical coefficient, and the reference symbolsA, B, C and D designates the aspherical surface coefficients.

As is understood from the foregoing description, the zoom lens systemaccording to the present invention is designed for a high vari-focalratio of 2, and nevertheless has excellent optical performance and acompact design, and can be manufactured at a low cost.

I claim:
 1. A zoom lens system comprising in the order from the objectside: a front lens unit comprising a positive meniscus lens componenthaving a convex surface on the object side, a negative lens componentand at least one positive lens component, and having a positiverefractive power as a whole, and a rear lens unit comprising a positivelens component, a biconcave lens component and a negative meniscus lenscomponent having a convex surface on the image side, and having anegative refractive power as a whole; and adapted so as to performvariation of focal length by varying the airspace reserved between saidfront lens unit and said rear lens unit; the positive lens component andthe biconcave lens component arranged in said rear lens unit beingfabricated from plastic material and comprising at least one asphericalsurface having positive refractive power strengthened as the surfaceportions are farther from the optical axis, and said rear lens unitbeing designed so as to satisfy the following conditions (1) and (2):

    -1 <(r.sub.II2a +r.sub.II2b)/(r.sub.II2a -r.sub.II2b) <0.2 (1)

    -0.15 21 f.sub.II /f.sub.II12 <0.15                        (2)

wherein the reference symbols r_(II2a) and r_(II2b) represent the radiiof curvature on the object side surface and the image side surfacerespectively of the biconcave lens component arranged in said rear lensunit, the reference symbol f_(II) designates the focal length of saidrear lens unit, and the reference symbol f_(II12) denotes the totalfocal length of two lens components fabricated from plastic material. 2.A zoom lens system according to claim 1 wherein said rear lens unit isdesigned so as to satisfy the following condition (2'):

    -0.15 <f.sub.II /f.sub.II12 <0.03                          (2')


3. A zoom lens system according to claim 1 or 2 wherein said two plasticlens components are fabricated from one and the same substance, and saidrear lens unit is designed so as to satisfy the following conditions(3), (4) and (5):

    1 ≧|f.sub.II1 /f.sub.II |≧4 (3)

    1 ≧|f.sub.II2 /f.sub.II |≧4 (4)

    -0.8 <(r.sub.IIb -r.sub.II2a)/(r.sub.II1b +r.sub.II2a) <0  (5)

wherein the reference symbol f_(II1) represent the focal length of thepositive plastic lens component arranged in said rear lens unit, thereference symbol f_(II2) designates the focal length of the biconcaveplastic lens component arranged in said rear lens unit and the referencesymbol r_(II1b) denotes the radius of curvature on the image sidesurface of the positive plastic lens component arranged in said rearlens unit.
 4. A zoom lens system according to claim 3 wherein said frontlens unit comprises, in the order from the object side, a positive lenscomponent having a convex surface on the object side, a negative lenscomponent and two positive lens components both having convex surfaceson the image side, and is designed so as to satisfy the followingcondition (6):

    -2.4 <(r.sub.I2b -r.sub.I3a)/(r.sub.I2b +r.sub.I3a)        (6)

wherein the reference symbol r_(I2b) represents the radius of curvatureon the image side surface of the negative lens component arranged insaid front lens unit, and the reference symbol r_(I3a) designates theradius of curvature on the object side surface of the positive lenscomponent arranged on the image side of the negative lens component andadjacent thereto in said front lens unit.
 5. A zoom lens systemaccording to claim 4 wherein said front lens unit is designed so as tosatisfy the following condition (6'):

    -2.4 <(r.sub.I2b -r.sub.I3a)/(r.sub.I2b +r.sub.I3a) <-0.5  (6')